Well I installed it, by just doing a “apt install mopidy” (or something), without getting any errors. Then I (based on my previous being used to mpd and because of various tutorials) wanted to configure my library and have the directory scanned, I did a “mopidy local scan” instead of a “mopidyctl local scan” and there it seems permission problems hit me (scanning the directory as root?) and not getting /etc/mopidy/mopidy.conf being used also hit me 
Stuff did not work, and I got weird python stack traces.
To me getting the error message from my first posting here is kind of an incompatibility. Yes, if you did the same thing to e.g. an smtp server, it probably would fail in a similar way, but then with most smtp servers there is only one way (systemwide as a daemon) to run them and various wrapper scripts take care about the user not stomping about permissions in a stupid way Yes, of course, you can call this user stupidity, not “two incompatible ways to use the software” and of course you are right. Basically what I am saying is: Why am I not presented with the “dpkg-reconfigure mopidy” debconfig question upon installation, why do I have to find out about it by getting errors?
Now I understand the hard place you are in, in respect to documenting and preconfiguring. Software like mopidy is underlying plumbing, that has lots of moving parts and flexibility, and there is really not “one right way” to use it, and taking away flexibility is not an option at all. Everybody comes from a different starting point. I would probably still be running mpd, if it were not for the home assistant integration, that just works for mopidy, not for mpd. Others probably could not care less about the way I run this software and are not interested in homeassistant at all (but Volumio-like setups or whatever).
So sorry for my verbose ramblings, I don’t know what to suggest either, except maybe for item b) but I will try to
a) get the Debian packager to maybe document slightly more prominently that mopidy can be run both as a system service or a user process (and you really have to decide about that or acknowledge this so you do not botch permissions).
b) get it somewhere into documentation that if you want to run the mopidy+snapcast combo, and want to use a fifo (don’t know if this is preferable, but it is what most documentation suggests it that way, some documentation says it gives less latency than tcp) that you have to do the fs.protected_fifos = 0 setting in /etc/sysctl.conf on modern(?) kernels.
c) Try to get the dead links on Snapcast | snapcast fixed
(like https://mjaggard.github.io/snapcast/doc/player_setup.md#mpd ). Not your problem at all, but small things like this make you go nuts, when you feel a bit lost in installing software anyway,
I’ve also found another problem in my setup, but I will start a seperate posting for this, my ramblings are already more incoherent than they should be.
Again thanks for your patience!
Ralph